Why is cylinder 4 misfiring on 2001 Audi A6 turbo?

Misfiring ignition components are usually the culprit, although bad gasoline, or the wrong octane fuel in the tank could also cause this symptom. Replace your spark plugs, and make sure they are properly gapped with your feeler gauge to OEM specifications. Check that your ignition coil is fully functional, as well as your spark plug wires by using a spark tester.